1. Field of the Invention
This invention relates to electrical devices comprising conductive polymers, and electrical circuits comprising such devices.
2. Introduction to the Invention
Conductive polymer compositions which exhibit PTC (positive temperature coefficient of resistance) behavior are well-known for use in electrical devices such as circuit protection devices and heaters. Such compositions with low resistivity are desirable for use in circuit protection devices which respond to changes in ambient temperature and/or current conditions. Under normal conditions, a circuit protection device remains in a low temperature, low resistance state in series with a load in an electrical circuit. When exposed to an overcurrent or overtemperature condition, however, the device increases in resistance, effectively shutting down the current flow to the load in the circuit. For many applications it is desirable that the device have as low a resistance as possible in order to minimize the effect on the resistance of the electrical circuit during normal operation. Furthermore, a low resistance allows the device to have a higher hold current, i.e. the largest steady state current that, under specified ambient conditions, can be passed through a circuit protection device without causing the device to "trip" into the high resistance state. Although low resistance devices can be made from a given composition by changing dimensions, e.g. making the distance between the electrodes very small or the device area very large, small devices are preferred. Such devices occupy less space on a circuit board and generally have desirable thermal properties. The most common technique to achieve a small device is to use a composition that has a low resistivity.